Printing most typically involves transferring ink from an inked image on a printing plate or other imaged printing medium to a suitable substrate. Printing is often performed using a printing press in which a printing plate on a rotating print cylinder is brought into contact with a substrate to be printed on.
There are a great many parameters that affect the quality of a printed image. Under some circumstances it can be advantageous to rotate an image slightly relative to the print cylinder. For example, small rotations of images on a print cylinder, such as rotations on the order of one degree, can be used to reduce ghosting. Ghosting is the occurrence of unwanted areas of higher or lower ink density. Ghosting results from interactions between the layout of an image being printed and the inking system of a printing press. Ghosting is particularly noticeable in areas which include a boundary between an uninked area and an inked area which extends parallel to the direction of substrate travel.
In film-based printing, skilled press operators will sometimes place films at an angle to printing plates while exposing the printing plates in order to reduce ghosting problems. This is typically done after examination of a film or a proof by someone who has experience with this type of printing problem. In many modern computer-to-plate printing systems or in printing presses in which the image is carried on a printing sleeve there is no film and so this approach is not possible. Instead, images must be digitally rotated. The rotated images can then be imparted to the printing plate or sleeve.
U.S. Pat. No. 6,441,914 discloses a computerized prepress system which includes a facility for automatically detecting situations in which ghosting is likely to occur and electronically angling an image and/or inserting ghosting prevention bars to reduce or eliminate ghosting. This patent does not describe how to accomplish image rotation.
Various image rotation algorithms exist. Many such algorithms are unsuitable for use in a printing environment. Some existing methods for rotating an image may introduce various visible artifacts or distort the printed image unacceptably. Further, some prior art image rotation algorithms are memory intensive, computationally intensive and/or I/O intensive. The use of such algorithms is undesirably inefficient. Such algorithms can may also be impractical or even impossible to implement within the hardware and software constraints of certain printing systems.
In view of the foregoing, there is a need for practical systems and methods for rotating images through small angles. It is desirable that such practical systems not significantly slow the transmission of image data to an imaging device.